Polyamidimide and polyimide are well known for their excellent thermal and electrical properties and have accordingly been widely used for magnet wire coatings and electronic packaging applications. One of the drawbacks of these materials is craze formation. Crazes initiate and propagate with or without the introduction of solvents. As crazes propagate, cracking across film layers often occurs. In the presence of varnish, crazing on polyamideimide film forms many ring-like crazes which are perpendicular to wires or coils. Crazing can also penetrate through base coat layers and expose the underlying wires, thus, resulting in electrical failure. In the field of magnet wires, electrical surge failure is frequently encountered after a varnishing process where water-borne epoxy resin is typically applied.
The mechanism which causes crazing remains unclear. Experimental data taken over the years suggests that crazing results from residual thermal stresses which occur during the curing process as a result of solvent evaporation, the extent of imidization, and the differences in the thermal expansion coefficients of the polyamideimide and the polyester base coats. As coating thicknesses increase, stress caused by thermal expansion increases and is amplified by rapid curing cycles that are commonly used in the fabrication of magnet wire.
On the molecular level, thermal stress is the result of the freezing of over-stretched polyamideimide chains, imperfect morphology, irregularly distributed density, and possibly gel particulates. Governed by the entropy of the system, in the presence of varnish, the over-stretched polyamideimide chains tend to recoil, resulting in shrinkage and, thus, craze formation on a macrometric scale.
Solvent or varnish-induced craze resistant polyamideimide enamel for magnet wire has become increasingly important since most modern winding processes tend to eliminate additional stress relief devices. The following U.S. patents exemplify various attempts to minimize crazing effects in wire coatings: U.S. Pat. No. 5,443,914 to Hart, et al., U.S. Pat. No. 5,567,781 to Martino, et al., U.S. Pat. No. 4,839,444 to Lavallee, and U.S. Pat. No. 4,181,686 to Vassiliou.
According to other features, characteristics, embodiments and alternatives of the present invention, which will become apparent as the description thereof proceeds below, an illustrative embodiment of the present invention provides a craze resistant wire coating composition for magnet wires which includes:
(a) a compound selected from the group consisting of diacid compounds, trimellitic anhydrides, benzophenonetetracarcoxylic anhydrides and mixtures thereof;
(b) a diisocyanate compound;
(c) a dihydroxyl compound;
(d) a trihydroxyl compound;
(e) a fluoropolymer; and
(f) a mineral filler.
The present invention further provides a magnet wire which comprises:
a conductive element; and
a layer of a craze resistant wire coating composition on the conductive element that comprises:
(a) a compound selected from the group consisting of diacid compounds, trimellitic anhydrides, benzophenonetetracarcoxylic anhydrides and mixtures thereof;
(b) a diisocyanate compound;
(c) a dihydroxyl compound;
(d) a trihydroxyl compound;
(e) a fluoropolymer; and
(f) a mineral filler.